The invention relates to a method of purifying a methyl-diethanolamine aqueous solution.
Methyldiethanolamine aqueous solutions are used to remove hydrogen sulfide by absorption from gases, which have been produced by gasification of carbonaceous material, especially fine grained to powdery coal.
The gases of the above-described type contain impurities including particularly ammonia (NH.sub.3), hydrogen cyanide (HCN), hydrogen sulfide (H.sub.2 S) and carbonyl sulfide (COS). Since these impurities disturb further processing or work-up of these gases, they must be removed from these gases in a suitable manner. Several different methods for doing this are already known. Usually the ammonia present in these gases can be nearly completely removed and the hydrogen cyanide and the hydrogen sulfide can be partially removed by washing with a circulating scrubbing or wash liquid. The residual ammonia, the hydrogen cyanide and the hydrogen sulfide are separated subsequently in a separate separating column from the circulating scrubbing or wash liquid. The aqueous solutions of methyldiethanolamine, subsequently the so-called MDEA-solution, have proven particularly suitable for removal of the hydrogen sulfide present in these gases. The hydrogen sulfide is absorbed in the MDEA-solution and can subsequently be separated from the solution containing the impurities.
In practice it has been shown that, as a result of the HCN content present during the course of the process, formate is formed and builds up in the MDEA solution. The formate is formed by hydrolysis of HCN to formic acid and subsequently the formic acid is converted to the amine. However the absorptivity of the MDEA-solution for hydrogen sulfide is generally believed to be impaired by the formate formation.